1. Field of the Invention
The present invention relates to an ignition detecting device for detecting the presence or absence of ignition spark produced by an ignition apparatus for internal combustion engine mainly used in automobiles.
2. Description of the Related Art
With recent development of electronic implementation of fuel injection apparatuses, ignition apparatuses, exhaust gas control apparatuses and the like, it has been emphasized that an adverse influence of these apparatuses on an exhaust gas, when they are in failure, should be taken into careful consideration.
This type of apparatus should function to determine whether ignition spark is produced normally in combustion chambers of an internal combustion engine. Therefore, several proposals have hitherto been made to determine whether normal ignition spark is produced or not by electronically analyzing a high voltage output waveform on the secondary side of an ignition coil (for example, U.S. Pat. No. 3,942,102 specification), to detect a flyback voltage produced at the collector of an output transistor for turning on and off a primary winding current of an ignition coil (for example, JP-A-56-143326), etc.
The former proposal can detect accurately the presence or absence of the occurrence of ignition spark, however, since it detects a high voltage output waveform appearing on the secondary side of the ignition coil, sufficient insulation becomes necessary for the path through which the secondary side high voltage is introduced into an electronic circuit, and, particularly, there has been a problem that application of this proposal to vehicle-mounted apparatuses is difficult from the viewpoint of the structure and cost.
The latter proposal, on the other hand, detects a flyback voltage produced at the collector of an output transistor for turning on and off a primary winding current of the ignition coil, so that it does not require to introduce a secondary side high voltage into the electronic circuit. However, there is a problem that, whenever a high voltage is produced at the secondary side of the ignition coil, a flyback voltage is generated even in the absence of ignition spark, so that, even when the ignition plug requires a voltage higher than the secondary side voltage of the ignition coil or even when a high tension cord is disconnected and hence no ignition spark is generated at the ignition plug, an erroneous detection is made.